Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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123 FXUS61 KPHI 210146 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 946 PM EDT Sun Aug 20 2017 .SYNOPSIS... High pressure over the area moves offshore tonight, then slowly drifts out to sea through Tuesday. A cold front approaches from the west on Tuesday, then works its way across the region on Wednesday. Canadian high pressure builds into the region late in the week and into next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Relatively tranquil weather conditions will persist through tonight as high pressure remains over the area. Scant cloud cover and light and variable winds will lead to favorable radiational cooling tonight, with temperatures falling to near or within a few degrees of climatological normals. Dew points have increased a bit more than previously forecast, so expanded the mention of patchy fog, though agree with the previous shift that any fog will be shallow radiation fog and will likely be short lived. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... As high pressure shifts offshore, flow will return to a more southerly direction allowing warm/moist advection through the afternoon. Temperatures will be a few degrees higher than Sunday, while humidity will markedly increase towards the evening. There may be a chance for some showers and thunderstorms as a short wave trough over the Midwest moves eastward, reaching the area by afternoon/evening. One limiting factor will be rather diffuse surface boundaries - with little to no forecast convergence expected to focus or enhance activity. As such, the best chance for any development will be along higher terrain, mainly to the west of Philadelphia. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Some mid-level shortwave energy will pass through the region Monday night. Onshore flow will usher a warm and humid airmass into the region, as surface dewpoints will climb into the upper 60s to low 70s, and into the mid 70s across the Delmarva and extreme southern NJ. This will result in isolated to scattered showers and thunderstorms in the evening, gradually tapering off after midnight. Patchy fog may form in the pre-dawn hours going into Tuesday morning. On Tuesday, low pressure over the Great Lakes will lift to the northeast towards eastern Canada. This will drag a cold front through the Great Lakes and into the Ohio Valley late Tuesday. Pre-frontal trough sets up ahead of the front, and that looks to trigger afternoon and evening showers and thunderstorms. Abundant low level moisture will be in place with dewpoints in the low to mid 70s, and with several strong shortwaves out ahead of the upper trough/surface cold front, can expect some fairly strong thunderstorms during the overnight hours Tuesday night into Wednesday morning. Cold front works its way to the south and east Wednesday through Wednesday evening, then becomes nearly stationary over the Mid-Atlantic and Delmarva for the end of the week. Quite warm and humid for the first half of the long term period, with highs in the upper 80s to low 90s on Tuesday, then in the mid to upper 80s on Wednesday. Max heat index values on Tuesday look to be in the upper 90s for areas along and south of the I-95 corridor, and around 100 for the Delmarva. Several waves of low pressure may develop on this boundary, but latest model guidance keeps those systems well offshore. Canadian high pressure then builds in from the north and west to close out the work week, with the center of the high gradually passing north of the region on Saturday before moving offshore on Sunday. Dry conditions expected for the end of the week and into the weekend with temperatures running several degrees below normal. Highs will top off in the upper 70s to around 80 from Thursday through Sunday, and lows will drop into the upper 50s to low 60s, except cooler up north. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions. There is a low chance of patchy fog development towards daybreak tonight but with drier conditions expect any development to be limited. Westerly winds through this afternoon will gradually become light southwesterly then light and variable as high pressure shifts across the area. By Monday, high pressure will shift offshore allowing southerly flow to strengthen to around 5-10 kt through the afternoon. There is an increasing chance for showers and thunderstorms after 18Z, generally 20 to 30 percent for all TAF sites. However, the chance is too low to include in the TAFs at this time. Outlook... Monday night...Sub-VFR conditions possible in scattered SHRA/TSRA. Fog/stratus also possible with sub-VFR conditions. Tuesday...Morning fog/stratus burns off, then predominantly VFR. Scattered SHRA/TSRA develop late in the day and at night with sub-VFR conditions. Wednesday...Sub-VFR conditions in showers/thunderstorms, which taper off late in the day. A wind shift from S to NW likely late in the day with the passage of a cold front. Thursday through Friday...VFR and light winds. && .MARINE... Winds and seas remain tranquil across the waters through Monday as high pressure gradually shifts offshore. Seas will remain two feet or less through Monday afternoon, with a few 3 foot seas possible off the northern NJ coast by evening as southerly winds begin to strengthen. Outlook... Monday night...Winds and seas below SCA thresholds. Tuesday...SCA possible late afternoon and night with S winds increasing to around 25 kt ahead of cold front. Building seas to 4-6 ft are forecast for the Atlantic coastal waters. Wednesday...Winds should decrease to below 25 kt but SCA conditions may continue into the morning as it may take longer for seas in the coastal waters to drop below 5 ft. Thursday through Friday...No marine hazards expected. Rip currents... At this point we expect the risk for the formation of dangerous rip currents on Monday to begin low. However, with winds increasing out of the south and the dominant period trending longer, the risk could increase as we go through the day depending on how quickly the wind shift occurs. For now we will mention a low risk in the surf forecast but continue to monitor closely especially as we go through the day on Monday. && .TIDES/COASTAL FLOODING... Minor coastal flooding is possible at the high tide Monday evening. With high pressure offshore, onshore will winds develop, and this will coincide with relatively high astronomical tides due to a new moon. Latest guidance is showing water levels just touching minor coastal flooding thresholds. However, water levels with the past three high tide cycles have been higher than most guidance, thus will include a mention in the hazardous weather outlook for the potential of minor tidal flooding. Will wait to make a decision on an advisory until we get the next run of guidance in and have a chance to run local regression equations based on the tidal departures we saw this evening. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...Johnson/99 Short Term...99 Long Term...MPS Aviation...Johnson/MPS/99 Marine...Johnson/MPS/99 Tides/Coastal Flooding...

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